Maize seedling morphology and defence hormone profiles,but not herbivory tolerance,were mediated by domestication and modern breeding

We addressed whether Zea seedling morphology relevant to performance, defence hormone profiles and tolerance of a phloem‐feeding, specialist herbivore were affected by two processes, plant domestication and modern breeding. Domestication effects were inferred through comparisons between Balsas teosintes (Zea mays parviglumis) and landrace maizes (Z. mays mays), and modern breeding effects through comparisons between landrace maizes and inbred maize lines. Specifically, we compared seedling forms (a composite measure of leaf length, average stem diameter, shoot wet weight, shoot dry weight, total root length, root wet weight, and root dry weight), shapes (forms scaled by seedling dry weight, a proxy for seedling size), and defence hormone profiles among Balsas teosinte and landrace and inbred line maizes, exposed or unexposed to feeding by Dalbulus maidis. Our results suggested that domestication as well as modern breeding strongly mediated both seedling form and shape. Form was more similar between landrace and inbred maize than between Balsas teosinte and landrace maize, suggesting that domestication affected seedling form more than modern breeding. In contrast, shape was more similar between Balsas teosinte and landrace maize than between landrace and inbred maizes, suggesting that modern breeding affected seedling shape more than domestication. Additionally, seedling shoot : root ratios appeared to have been mediated by domestication, but not by modern breeding. In broad terms, individual seedling structures relevant to seedling ecology in wild or managed environments, such as leaf and root lengths, and shoot and root masses, were enlarged with domestication and reduced with modern breeding. Herbivory did not affect seedling shape, but had a weak effect on form so that seedlings were slightly larger in the absence versus presence of D. maidis. Also, both domestication and modern breeding seem to have mediated seedling hormone profiles, with breeding more strongly mediating profiles than domestication. Jasmonic acid isoleucine (JA‐Ile) and salicylic acid (SA) were induced by herbivory in both teosinte and maize. The hormone profiles assays collectively suggested that domestication and modern breeding altered constitutive levels of SA, abscisic acid and JA‐related (JA‐Ile and oxo‐phytodienoic acid) hormone levels in seedlings, particularly by increasing the levels of SA and decreasing those of JA‐related hormones. Altogether, our results suggested that maize domestication and modern breeding significantly altered seedling form, shape, ecologically relevant morphological traits (e.g. leaf and root lengths, and shoot and root masses) and hormonal defences, but not tolerance of D. maidis herbivory.     

Cellular differentiation in root caps of Zea mays that do not secrete mucilage

We quantified the structural changes accompanying cellular differentiation in root caps of Zea mays cv. Ageotropic to determine the developmental basis for the nongraviresponsiveness of their primary roots. Cells of the calyptrogen and columella of primary roots of the ageotropic mutant have structures indistinguishable from those of caps of primary roots of Z. mays cv. Kys the graviresponsive, wild-type parent of Z. mays cv. Ageotropic. However, the relative volumes of dictyosomes, dictyosome-derived vesicles and starch in the outermost peripheral cells of wild-type roots were significantly lower than were those in peripheral cells of mutant roots. This corresponds to a dramatic accumulation of starch and mucilage-filled vesicles in peripheral cells of mutant roots. Cellular differentiation in root caps of graviresponsive seminal roots of the Ageotropic mutant resembled that of primary and seminal roots of the wild-type cultivar, and differed significantly from that of primary roots of the mutant. We conclude that the mutation that blocks secretion of mucilage from peripheral cells of Ageotropic roots: (1) expresses itself late in cellular differentiation in root caps; (2) is expressed only in primary (but not seminal) roots of the Ageotropic mutant; and (3) is consistent with malfunctioning dictyosomes and dictyosome-derived vesicles being the cellular basis for agravitropism of primary roots of this mutant.     

Plastome genomics in South American maize landraces: chloroplast lineages parallel the geographical structuring of nuclear gene pools

Background and AimsThe number of plastome sequences has increased exponentially during the last decade. However, there is still little knowledge of the levels and distribution of intraspecific variation. The aims of this study were to estimate plastome diversity within Zea mays and analyse the distribution of haplotypes in connection with the landrace groups previously delimited for South American maize based on nuclear markers.MethodsWe obtained the complete plastomes of 30 South American maize landraces and three teosintes by means of next-generation sequencing (NGS) and used them in combination with data from public repositories. After quality filtering, the curated data were employed to search for single-nucleotide polymorphisms, indels and chloroplast simple sequence repeats. Exact permutational contingency tests were performed to assess associations between plastome and nuclear variation. Network and Bayesian phylogenetic analyses were used to infer evolutionary relationships among haplotypes.Key ResultsOur analyses identified a total of 124 polymorphic plastome loci, with the intergenic regions psbE-rps18, petN-rpoB, trnL_UAG-ndhF and rpoC2-atpI exhibiting the highest marker densities. Although restricted in number, these markers allowed the discrimination of 27 haplotypes in a total of 51 Zea mays individuals. Andean and lowland South American landraces differed significantly in haplotype distribution. However, overall differentiation patterns were not informative with respect to subspecies diversification, as evidenced by the scattered distribution of maize and teosinte plastomes in both the network and Bayesian phylogenetic reconstructions.ConclusionsKnowledge of intraspecific plastome variation provides the framework for a more comprehensive understanding of evolutionary processes at low taxonomic levels and may become increasingly important for future plant barcoding efforts. Whole-plastome sequencing provided useful variability to contribute to maize phylogeographic studies. The structuring of haplotype diversity in the maize landraces examined here clearly reflects the distinction between the Andean and South American lowland gene pools previously inferred based on nuclear markers.     

Detection of quantitative trait loci for seminal root traits in maize (Zea mays L.) seedlings grown under differential phosphorus levels

Suboptimal phosphorus availability is a primary constraint for terrestrial plant growth. Seminal roots play an important role in acquisition of nutrients by plant seedlings. The length and number of seminal roots may be particularly important in acquisition of immobile nutrients such as phosphorus by increasing soil exploration. The objective of this study was to identify quantitative trait loci (QTL) controlling seminal root growth in response to phosphorus stress in maize, and to characterize epistatic interactions among QTL. Seminal root length and number were evaluated in 162 recombinant inbred lines derived from a cross between B73 and Mo17 in seedlings grown in a controlled environment. B73 and Mo17 significantly differed for seminal root length under low phosphorus, but not under adequate phosphorus conditions. Seminal root length of the population grown under low phosphorus ranged from 0 to 79.2 cm with a mean of 32.3 cm; while seminal root length of plants grown under high phosphorus ranged from 0.67 to 59.0 cm with a mean of 23.4 cm. Under low phosphorus, one main-effect QTL was associated with seminal root length and three QTL with seminal root number; under high phosphorus, two QTL with seminal root length and three QTL for seminal root number. These accounted for 11, 25.4, 22.8, and 24.1% of the phenotypic variations for seminal root length and number at low phosphorus, and seminal root length and number at high phosphorus, respectively. Di-genic epistatic loci were detected for seminal root length at low phosphorus (two pairs) seminal root number at low phosphorus (eight pairs), seminal root length at high phosphorus (four pairs), and seminal root number at high phosphorus (two pairs), which accounted for 23.2, 50.6, 32.2, and 20.3% of the total variations, respectively. Seminal root traits observed here were positively yet weakly correlated with shoot biomass in the field under low phosphorus, although no coincident QTL were detected. These results suggest that epistatic interactions are important in controlling genotypic variation associated with seedling seminal root traits.     

Genomic screening for artificial selection during domestication and improvement in maize

BACKGROUND: Artificial selection results in phenotypic evolution. Maize (Zea mays L. ssp. mays) was domesticated from its wild progenitor teosinte (Zea mays subspecies parviglumis) through a single domestication event in southern Mexico between 6000 and 9000 years ago. This domestication event resulted in the original maize landrace varieties. The landraces provided the genetic material for modern plant breeders to select improved varieties and inbred lines by enhancing traits controlling agricultural productivity and performance. Artificial selection during domestication and crop improvement involved selection of specific alleles at genes controlling key morphological and agronomic traits, resulting in reduced genetic diversity relative to unselected genes. SCOPE: This review is a summary of research on the identification and characterization by population genetics approaches of genes affected by artificial selection in maize. CONCLUSIONS: Analysis of DNA sequence diversity at a large number of genes in a sample of teosintes and maize inbred lines indicated that approx. 2 % of maize genes exhibit evidence of artificial selection. The remaining genes give evidence of a population bottleneck associated with domestication and crop improvement. In a second study to efficiently identify selected genes, the genes with zero sequence diversity in maize inbreds were chosen as potential targets of selection and sequenced in diverse maize landraces and teosintes, resulting in about half of candidate genes exhibiting evidence for artificial selection. Extended gene sequencing demonstrated a low false-positive rate in the approach. The selected genes have functions consistent with agronomic selection for plant growth, nutritional quality and maturity. Large-scale screening for artificial selection allows identification of genes of potential agronomic importance even when gene function and the phenotype of interest are unknown. These approaches should also be applicable to other domesticated species if specific demographic conditions during domestication exist.     

Population genetic structure of a specialist leafhopper on Zea: likely anthropogenic and ecological determinants of gene flow

Corn leafhopper, Dalbulus maidis DeLong & Wolcott (Hemiptera: Cicadellidae), is a specialist herbivore on the genus Zea (Poaceae). The genera Dalbulus and Zea evolved in central Mexico. We sought to determine whether population genetic structuring is prevalent in corn leafhoppers inhabiting three of its host plants: (1) the highland species perennial teosinte (Zea diploperennis Iltis, Doebley & Guzman), (2) the mid‐ to lowland‐species Balsas teosinte (Zea mays ssp. parviglumis Iltis & Doebley), and (3) the ubiquitous domesticated maize (Zea mays ssp. mays L.). We used amplified fragment length polymorphisms to detect population structuring and genetic differentiation among corn leafhoppers on the three host plants in western‐central and ‐northern Mexico. Our results showed that corn leafhopper in Mexico is composed of at least two genetically discrete populations: an ‘Itinerant’ population associated with the annual hosts maize and Balsas teosinte, which appears to be widely distributed in Mexico, and a ‘Las Joyas’ population restricted to perennial teosinte and confined to a small mountain range (Sierra de Manantlán) in western‐central Mexico. Our results further suggested that population structuring is not due to isolation by distance or landscape features: Las Joyas and Itinerant corn leafhopper populations are genetically distinct despite their geographic proximity (ca. 4 km), whereas Itinerant corn leafhoppers separated by hundreds of kilometers (>800 km), mountain ranges, and a maritime corridor (Sea of Cortez) are not genetically distinct. Based on our results and on published ethnohistorical and archaeological data, we propose pre‐Columbian and modern scenarios, including likely ecological and anthropogenic influences, in which the observed genetic population structuring of corn leafhopper could have originated and could be maintained. Also, we hypothesize that after evolving on the lowland Balsas teosinte, corn leafhopper expanded its host range to include maize and then the highland perennial teosinte, following the domestication and spread of maize within the last 9 000 years.     

Heterogeneous nutrient supply promotes maize growth and phosphorus acquisition: additive and compensatory effects of lateral roots and root hairs

Background and AimsRoot proliferation is a response to a heterogeneous nutrient distribution. However, the growth of root hairs in response to heterogeneous nutrients and the relationship between root hairs and lateral roots remain unclear. This study aims to understand the effects of heterogeneous nutrients on root hair growth and the trade-off between root hairs and lateral roots in phosphorus (P) acquisition.MethodsNear-isogenic maize lines, the B73 wild type (WT) and the rth3 root hairless mutant, were grown in rhizoboxes with uniform or localized supply of 40 (low) or 140 (high) mg P kg⁻¹ soil.ResultsBoth WT and rth3 had nearly two-fold greater shoot biomass and P content under local than uniform treatment at low P. Significant root proliferation was observed in both WT and rth3 in the nutrient patch, with the WT accompanied by an obvious increase (from 0.7 to 1.2 mm) in root hair length. The root response ratio of rth3 was greater than that of WT at low P, but could not completely compensate for the loss of root hairs. This suggests that plants enhanced P acquisition through complementarity between lateral roots and root hairs, and thus regulated nutrient foraging and shoot growth. The disappearance of WT and rth3 root response differences at high P indicated that the P application reduced the dependence of the plants on specific root traits to obtain nutrients.ConclusionsIn addition to root proliferation, the root response to a nutrient-rich patch was also accompanied by root hair elongation. The genotypes without root hairs increased their investment in lateral roots in a nutrient-rich patch to compensate for the absence of root hairs, suggesting that plants enhanced nutrient acquisition by regulating the trade-off of complementary root traits.     

Formation of aerenchyma in roots of Zea mays in aerated solutions, and its relation to nutrient supply

The formation of lysigenous cavities (aerenchyma) in the root cortex of maize, Zea mays L. cv. Capella, under well-aerated conditions has been studied in relation to the composition of the nutrient solutions. Nitrogen, either supplied as nitrate or as ammonium, reduced the cavity formation by the roots. This reduction was most apparent at nitrate concentrations above 2 mM. Cavities were increasingly formed when the nitrate concentration was decreased and they reached their largest dimensions in roots growing in water. Thus, inadequate availability of nitrogen leads, under acrated conditions, to deterioration of cortex cells and cavity formation in the maize roots. It is suggested that cavity formation in these roots is connected with reduced nitrogen assimilation.     

Arbuscular mycorrhizal colonization outcompetes root hairs in maize under low phosphorus availability

Background and AimsAn increase in root hair length and density and the development of arbuscular mycorrhiza symbiosis are two alternative strategies of most plants to increase the root–soil surface area under phosphorus (P) deficiency. Across many plant species, root hair length and mycorrhization density are inversely correlated. Root architecture, rooting density and physiology also differ between species. This study aims to understand the relationship among root hairs, arbuscular mycorrhizal fungi (AMF) colonization, plant growth, P acquisition and mycorrhizal-specific P_i transporter gene expression in maize.MethodsUsing nearly isogenic maize lines, the B73 wild type and the rth3 root hairless mutant, we quantified the effect of root hairs and AMF infection in a calcareous soil under P deficiency through a combined analysis of morphological, physiological and molecular factors.Key ResultsWild-type root hairs extended the rhizosphere for acid phosphatase activity by 0.5 mm compared with the rth3 hairless mutant, as measured by in situ zymography. Total root length of the wild type was longer than that of rth3 under P deficiency. Higher AMF colonization and mycorrhiza-induced phosphate transporter gene expression were identified in the mutant under P deficiency, but plant growth and P acquisition were similar between mutant and the wild type. The mycorrhizal dependency of maize was 33 % higher than the root hair dependency.ConclusionsThe results identified larger mycorrhizal dependency than root hair dependency under P deficiency in maize. Root hairs and AMF inoculation are two alternative ways to increase P_i acquisition under P deficiency, but these two strategies compete with each other.     

Facilitated by nature and agriculture: performance of a specialist herbivore improves with host-plant life history evolution, domestication, and breeding

Plant defenses against herbivores are predicted to change as plant lineages diversify, and with domestication and subsequent selection and breeding in the case of crop plants. We addressed whether defense against a specialist herbivore declined coincidently with life history evolution, domestication, and breeding within the grass genus Zea (Poaceae). For this, we assessed performance of corn leafhopper (Dalbulus maidis) following colonization of one of four Zea species containing three successive transitions: the evolutionary transition from perennial to annual life cycle, the agricultural transition from wild annual grass to primitive crop cultivar, and the agronomic transition from primitive to modern crop cultivar. Performance of corn leafhopper was measured through seven variables relevant to development speed, survivorship, fecundity, and body size. The plants included in our study were perennial teosinte (Zea diploperennis), Balsas teosinte (Zea mays parviglumis), a landrace maize (Zea mays mays), and a hybrid maize. Perennial teosinte is a perennial, iteroparous species, and is basal in Zea; Balsas teosinte is an annual species, and the progenitor of maize; the landrace maize is a primitive, genetically diverse cultivar, and is ancestral to the hybrid maize; and, the hybrid maize is a highly inbred, modern cultivar. Performance of corn leafhopper was poorest on perennial teosinte, intermediate on Balsas teosinte and landrace maize, and best on hybrid maize, consistent with our expectation of declining defense from perennial teosinte to hybrid maize. Overall, our results indicated that corn leafhopper performance increased most with the agronomic transition, followed by the life history transition, and least with the domestication transition.     

Immunohistological localization and quantification of IAA in studies of root growth regulation

The content and distribution of auxins were studied in gravistimulated roots of maize (Zea mays L.) and primary roots of 7-day-old wheat (Triticum durum Desf.) seedlings, which branching was enhanced by excision of adventitious roots. IAA localization was observed immunohistochemically, using specific anti-IAA antibody in combination with second (anti-species) antibody labeled with colloidal gold. Differences in the IAA content (staining intensity) were found between upper and lower parts of gravistimulated maize roots. We also observed IAA accumulation in the primary wheat root after adventitious root excision; the cells of lateral root primordia were characterized by more intense IAA staining. The role of auxin redistribution in plants for lateral root initiation and development is discussed.     

Root hair phenotypes influence nitrogen acquisition in maize

Background and AimsThe utility of root hairs for nitrogen (N) acquisition is poorly understood.MethodsWe explored the utility of root hairs for N acquisition in the functional–structural model SimRoot and with maize genotypes with variable root hair length (RHL) in greenhouse and field environments.Key ResultsSimulation results indicate that long, dense root hairs can improve N acquisition under varying N availability. In the greenhouse, ammonium availability had no effect on RHL and low nitrate availability increased RHL, while in the field low N reduced RHL. Longer RHL was associated with 216 % increase in biomass and 237 % increase in plant N content under low-N conditions in the greenhouse and a 250 % increase in biomass and 200 % increase in plant N content in the field compared with short-RHL phenotypes. In a low-N field environment, genotypes with long RHL had 267 % greater yield than those with short RHL. We speculate that long root hairs improve N capture by increased root surface area and expanded soil exploration beyond the N depletion zone surrounding the root surface.ConclusionsWe conclude that root hairs play an important role in N acquisition. We suggest that root hairs merit consideration as a breeding target for improved N acquisition in maize and other crops.     

Plant defense against fall armyworm in micro‐sympatric maize (Zea mays ssp. mays) and Balsas teosinte (Zea mays ssp. parviglumis)

Maize [Zea mays L. ssp. mays (Poaceae)] was domesticated from Balsas teosinte (Zea mays ssp. parviglumis Iltis & Doebley) in present‐day Mexico. Fall armyworm, Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae), is among the most important pests of maize in Mexico and Central America. We compared the strength of plant defenses against fall armyworm between micro‐sympatric landrace maize and Balsas teosinte in the field and laboratory. The field comparison, conducted in Mexico, consisted of comparing the frequency of fall armyworm infestation between young maize and Balsas teosinte plants in dryland agricultural fields in which Balsas teosinte grew as a weed. The laboratory comparison contrasted the performance of fall armyworm larvae provided a diet of leaf tissue excised from maize or Balsas teosinte plants that were intact or had been primed by larval feeding. In the field, maize plants were more frequently infested with fall armyworm than Balsas teosinte plants: over 3 years and three fields, maize was infested at a ca. 1.8‐fold greater rate than Balsas teosinte. In the laboratory, larval growth, but not survivorship, was differently affected by feeding on maize vs. Balsas teosinte, and on primed vs. intact plants. Specifically, survivorship was ca. 98%, and did not differ between maize and Balsas teosinte, nor between primed and intact plants. Larvae grew less on intact vs. primed maize, and similarly on intact vs. primed Balsas teosinte; overall, growth was 1.2‐fold greater on maize compared to Balsas teosinte, and on primed compared to intact plants. Parallel observations showed that the differences in growth could not be attributed to the amount of leaf tissue consumed by larvae. We discuss our results in relation to differences in the strength of plant defenses between crops and their ancestors, the relevance of unmanaged Balsas teosinte introgression in the context of fall armyworm defenses in maize, and whether greater growth of larvae on primed vs. intact plants signifies herbivore offense.     

Genetic dissection of root formation in maize (Zea mays) reveals root-type specific developmental programmes

BACKGROUND: Maize (Zea mays) forms a complex root system comprising embryonic and post-embryonic roots. The embryonically formed root system is made up of the primary root and a variable number of seminal roots. Later in development the post-embryonic shoot-borne root system becomes dominant and is responsible together with its lateral roots for the major portion of water and nutrient uptake. Although the anatomical structure of the different root-types is very similar they are initiated from different tissues during embryonic and post-embryonic development. Recently, a number of mutants specifically affected in maize root development have been identified. These mutants indicate that various root-type specific developmental programmes are involved in the establishment of the maize root stock. SCOPE: This review summarizes these genetic data in the context of the maize root morphology and anatomy and gives an outlook on possible perspectives of the molecular analysis of maize root formation.     

Alkaline cooking and tortilla quality in maize grains from the humid,tropical lands of Mexico

Maize (Zea mays L.) tortilla is the major staple food for the Mexican population. Nine tropical maize genotypes were evaluated. All samples had white grains, a common characteristic in tropical maize, and therefore they were appropriate for nixtamalized flour industry. Grain, flour, masa and tortilla characteristics of each maize genotype were evaluated. Length, width, thickness, weight of 1000 grains and hardness of grain were determined. Moisture content, proteins, fat, ash, mean particle size, water absorption index, enthalpy, and flour temperature were also evaluated. Adhesiveness and cohesiveness were evaluated in masa. Moisture content, protein, capacity to puff up, roll making, tension and cutting strength were determined in tortillas. There were significant differences (p≤0.05) in most of the evaluated characteristics. Grain length values varied between 9.26 and 11.02 mm for populations 23 and 22, respectively. Grain hardness oscillated between 11.17 (population 32) and 14.75 (landrace Mejen). According to the weight of 1000 grains most genotypes had small grains. The minimum and maximum moisture values of flour and tortillas were 8.33-9.99% and 46.20-50.36%, respectively. The texture of tortillas elaborated from population 32 and landrace Mejen had the lowest tension and cutting strength, resulting the best genotypes for making tortilla.     

Theoretical evidence that root penetration ability interacts with soil compaction regimes to affect nitrate capture

Background and AimsAlthough root penetration of strong soils has been intensively studied at the scale of individual root axes, interactions between soil physical properties and soil foraging by whole plants are less clear. Here we investigate how variation in the penetration ability of distinct root classes and bulk density profiles common to real-world soils interact to affect soil foraging strategies.MethodsWe utilize the functional–structural plant model ‘OpenSimRoot’ to simulate the growth of maize (Zea mays) root systems with variable penetration ability of axial and lateral roots in soils with (1) uniform bulk density, (2) plow pans and (3) increasing bulk density with depth. We also modify the availability and leaching of nitrate to uncover reciprocal interactions between these factors and the capture of mobile resources.Key ResultsSoils with plow pans and bulk density gradients affected overall size, distribution and carbon costs of the root system. Soils with high bulk density at depth impeded rooting depth and reduced leaching of nitrate, thereby improving the coincidence of nitrogen and root length. While increasing penetration ability of either axial or lateral root classes produced root systems of comparable net length, improved penetration of axial roots increased allocation of root length in deeper soil, thereby amplifying N acquisition and shoot biomass. Although enhanced penetration ability of both root classes was associated with greater root system carbon costs, the benefit to plant fitness from improved soil exploration and resource capture offset these.ConclusionsWhile lateral roots comprise the bulk of root length, axial roots function as a scaffold determining the distribution of these laterals. In soils with high soil strength and leaching, root systems with enhanced penetration ability of axial roots have greater distribution of root length at depth, thereby improving capture of mobile resources.     

Root cortical anatomy is associated with differential pathogenic and symbiotic fungal colonization in maize

Root anatomical phenotypes vary among maize (Zea mays) cultivars and may have adaptive value by modifying the metabolic cost of soil exploration. However, the microbial trade‐offs of these phenotypes are unknown. We hypothesized that nodal roots of maize with contrasting cortical anatomy have different patterns of mutualistic and pathogenic fungal colonization. Arbuscular mycorrhizal colonization in the field and mesocosms, root rots in the field, and Fusarium verticillioides colonization in mesocosms were evaluated in maize genotypes with contrasting root cortical anatomy. Increased aerenchyma and decreased living cortical area were associated with decreased mycorrhizal colonization in mesocosm and field experiments with inbred genotypes. In contrast, mycorrhizal colonization of hybrids increased with larger aerenchyma lacunae; this increase coincided with larger root diameters of hybrid roots. F. verticillioides colonization was inversely correlated with living cortical area in mesocosm‐grown inbreds, and no relation was found between root rots and living cortical area or aerenchyma in field‐grown hybrids. Root rots were positively correlated with cortical cell file number and inversely correlated with cortical cell size. Mycorrhizae and root rots were inversely correlated in field‐grown hybrids. We conclude that root anatomy is associated with differential effects on pathogens and mycorrhizal colonization of nodal roots in maize.     

The genetic architecture of nodal root number in maize

The maize nodal root system plays a crucial role in the development of the aboveground plant and determines the yield via the uptake of water and nutrients in the field. However, the genetic architecture of the maize nodal root system is not well understood, and it has become the ‘dark matter’ of maize genetics. Here, a large teosinte‐maize population was analyzed, and high‐resolution mapping revealed that 62 out of 133 quantitative trait loci (QTLs), accounting for approximately half of the total genetic variation in nodal root number, were derived from QTLs for flowering time, which was further validated through a transgenic analysis and a genome‐wide association study. However, only 16% of the total genetic variation in nodal root number was derived from QTLs for plant height. These results gave a hint that flowering time played a key role in shaping nodal root number via indirect selection during maize domestication. Our results also supported that more aerial nodal roots and fewer crown roots might be favored in temperate maize, and this root architecture might efficiently improve root‐lodging resistance and the ability to take up deep water and nitrogen under dense planting.     

Regulation of the maize (Zea mays L.) embryo proteome by RTCS which controls seminal root initiation

Seminal roots are initiated at the scutellar node during maize (Zea mays L.) embryo development. The maize mutant rtcs (rootless concerning crown and seminal roots) does not initiate seminal roots while its wild-type siblings form on average 2.9 seminal roots per seedling. In this study, proteome profiles of 25-day-old immature maize embryos were compared between wild-type and rtcs plants via two-dimensional electrophoresis (2-DE). Electrospray ionization tandem mass spectrometry (ESI-MS/MS) identified 23 proteins encoded by 21 different genes that were differentially accumulated between wild-type and rtcs embryos (Fc≥2; FDR<10%). Among the differentially accumulated proteins, two isoforms of a phosphoglycerate kinase and a malate dehydrogenase were preferentially accumulated in wild-type embryos. Both enzymes are related to the generation of energy-rich ATP or NADPH molecules and are crucial checkpoints of cellular energetics in plants. Comparison of embryonic proteins differentially accumulated between wild-type and rtcs embryos revealed little overlap with proteins differentially accumulated between wild-type and rum1 embryos which also do not initiate seminal roots. This might be due to distinct influences of RTCS and RUM1 on the composition of the embryo proteome, but could also be explained by different stages of embryo development that were analyzed in these studies.